1. Field of the Invention
The present invention relates generally to a control system for an automotive vehicle which has the capability of anti-skid control of a brake system and power distribution control for a power train which is selectively operable for adjusting driving torque distribution between front wheels and rear wheels. More specifically, the invention relates to technology for combining or correlating anti-skid control and power train control for obtaining optimal vehicle braking efficiency and enhanced stability.
2. Description of the Background Art
Japanese Patent First (unexamined) Publications (Tokkai) Showa 61-169361 and 61-295132 disclose anti-skid brake control systems for four-wheel drive type vehicles which have clutch mechanisms selectively switching the operation mode of a power train between a four-wheel drive mode and a two (front)-wheel drive mode depending upon the vehicle driving conditions. In such prior proposed systems, the anti-skid brake control system is so associated with a power train control system which switches the operation mode of the power train between the four-wheel drive mode and two-wheel drive mode, as to switch the operation mode from a four-wheel drive mode to a two-wheel drive mode for assured projection of vehicle body speed data based on wheel rotation speed of a wheel which is disconnected from the power source, i.e. an automotive engine, during an active state of the anti-skid brake control system.
As will be seen, the prior proposed system forcibly switches the power train operation mode from four-wheel drive mode to two-wheel drive mode irrespective of wheel slippage magnitude at a primary driving wheel which is constantly connected to the engine. In such a layout, the wheel disconnected from the engine is free from the deceleration torque of the engine during the active state of the anti-skid control. Namely, an engine braking torque is only active on the primary driving wheels. This implies that, because power distribution in the power train is 100 (%): 0 (%) between the primary driving wheels and auxiliary driving wheels which are disconnected from the engine during the active state of the anti-skid brake control, all of the negative driving torque is distributed to the driving wheels. This negative driving torque, i.e. engine braking torque, tends to cause delay in recovery of wheel rotation speed after occurrence of excessive wheel slippage. This tendency becomes substantially higher for a low friction road, such as a icy road, snow road and so forth. Delay of recovery of wheel rotation speed maintains the wheel slippage at an excessive level for a relatively long time so as to cause degradation of the vehicle drivability. In the case where the primary driving wheels are the rear wheels, vehicle driving stability can be degraded. On the other hand, in the case where the primary driving wheels are the front wheels, vehicle steering stability can also be degraded.
On the other hand, another approach has been proposed in SUBARU ALCYONE New Car Explanation, pages 108 and 161, published by Fuji Jukogyo Kabushiki Kaisha, on July, 1987. In this proposed technology, an engaging force of the transfer clutch is adjusted at a predetermined value during the active state of the anti-skid brake control system. Although this technology may slightly improve the vehicle driving stability, it still encounters a problem of vehicular driving instability particularly on a low friction road. Namely, even with the controlled engaging force, the deceleration caused by engine braking effect at relatively high vehicle speed on a low friction road may encounter the problem as in the former publications.